Hereafter, a wireless transmit/receive unit (WTRU) includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes, but is not limited to, a Node B, a site controller, an access point, or other interfacing device in a wireless environment.
Full downlink (DL) synchronization between a base station and a WTRU is obtained when the frame synchronization, code timing, and code locations are synchronized. Frame synchronization defines the beginning of a frame as seen by the WTRU receiver. Code timing is an integer multiple of the sampling period of the received signal in the WTRU receiver front end. Code location is the position of a path or multipath in time as observed by the WTRU receiver. Full synchronization is completed in three stages with three different algorithms: cell search (CS), automatic frequency control (AFC), and frame tracking (FT).
In the first stage, the CS algorithm finds the cell on which the WTRU is camped and performs frame synchronization based on the location of the first significant path (FSP) in the delay spread of a multipath channel. After the CS is completed, AFC commences. The AFC algorithm adjusts the code timing by adjusting the control voltage of the voltage controlled oscillator (VCO). Code timing is initially adjusted and also maintained by the AFC. When AFC is in its converged state (when the VCO operating frequency is adjusted), code locations are found by channel estimation. The output of channel estimation is code locations for every DL slot for the WTRU receiver.
Although CS performs the initial frame synchronization, there is still a need for maintenance of frame synchronization. Frame tracking is one way to maintain the DL frame synchronization of the WTRU. Since the frame synchronization is based on the location of the FSP, the frame tracking procedure is responsible for updating the FSP. The frame tracking procedure will run periodically after the initial frame synchronization.
If the frame tracking procedure does not run periodically, some paths at either end of the channel estimation vector of the WTRU may disappear, resulting in degraded performance due to loss of these paths. There are three main cases that may cause this to happen: WTRU motion, shadowing, and a fading multipath channel.
WTRU motion will result in a time shift of the channel estimation vector to either side depending on the initial and current positions of the WTRU. When the WTRU moves closer to the base station after being initially synchronized to it based on a particular distance, the propagation delay decreases. The FSP then appears earlier in time compared to the initial position. The paths will drift to the left of the channel estimation vector and will eventually disappear. The paths will drift in the opposite direction (i.e., towards the right edge of the channel estimation sequence vector), if the WTRU moves away from the base station. As long as frame synchronization with respect to the FSP position is updated, the channel estimation vector will show the FSP at or near the beginning of the channel estimation vector and all the paths in the delay spread will appear throughout the vector. As an example, for a WTRU radial velocity of 120 km/h, the drift of the FSP will happen very slowly, approximately a one-chip drift in 260 frames at 3.84 Mcps as used for 3GPP W-CDMA.
Multipath channel shadowing is another case where a frame synchronization update is required. During the initial synchronization of the WTRU, an object may block the direct path from the base station to the WTRU. When the blocking object or the WTRU changes position, the direct path may appear earlier than the currently known FSP and even earlier than the channel estimation window. To use this path, an FSP location update is required to provide frame synchronization.
Fading multipath channels are yet another challenge for frame synchronization. CS may not be able to detect the FSP successfully under multipath fading channel conditions. This situation may be avoided by using a longer accumulation period during the initial CS. However, due to a non-synchronized VCO and a limited time budget for initial frame synchronization, a number of frames of accumulations are performed that are insufficient in all cases to successfully find the position of the FSP.
For a multipath fading channel, the channel estimation must find the time locations and complex magnitudes of each path. A channel estimation algorithm should be able to follow the relatively slow and fast varying characteristics of the channel. One example of slowly varying channel characteristics is motion of a WTRU. Also, the difference of the frequencies of the WTRU and the base station local oscillators may result in a drift in the channel impulse response. When these effects are combined, they result in a drift in time in the channel impulse response.
Faster channel characteristic changes are due to the well-known multipath fading phenomena, which quickly makes dramatic changes in the magnitudes of the paths. The motion of the WTRU receiver affects all the paths in a similar manner. However, the multipath fading affects the paths in a unique manner by changing their power levels independently. Conventional channel estimation algorithms do not make use of these differences efficiently. This may result in excessive computations or lack of accuracy.
Multipath fading requires more frequent updates with higher resolution, where the frame of the data is fixed. For example, the commonly used RAKE receiver locates the position of the paths and tracks them individually by assigning a code tracker for each path. Meanwhile, coping with WTRU motion requires signal processing with less frequent updates and with lesser time resolution. These differences in update frequencies and resolutions are a challenge for channel estimation.